1. Field of the Invention
This invention generally relates to a device for detecting a ratio of an admixture or ingredient to a fuel mixture or mixed fuel to be supplied to an internal combustion engine or the like, and more particularly to a fuel mixing ratio detecting device for measuring a mixing ratio of alcohol to alcohol blended gasoline to be used in an automotive engine or the like.
2. Description of the Related Art
In recent years, in many countries such as the United States of America, European countries, Brazil, etc., alcohol blended gasoline has been introduced as a mixed fuel to be used in motor vehicles with the intention of reducing oil consumption and air pollution due to exhaust emissions of motor vehicles. When alcohol blended gasoline is used in an engine matched to an air fuel ratio of normal or non-blended gasoline, it becomes difficult to operate the engine. This is because the stoichiometric air fuel ratio of alcohol is smaller than that of gasoline and as a result, the air fuel ratio of alcohol blended gasoline becomes close to that of a lean mixture. Thus, the alcohol mixing ratio of the alcohol to the alcohol blended gasoline is first detected. Then, the air fuel ratio, the ignition timing and so on are controlled according to the detected alcohol mixing ratio.
A conventional device for detecting such a mixing ratio of the alcohol to the alcohol blended gasoline is disclosed in, for example, Japanese Patent Laid-Open No. 2-190755. This conventional device detects the dielectric constant .epsilon. of alcohol blended gasoline or fuel, namely, the alcohol mixing ratio thereof by utilizing a difference between the dielectric constant .epsilon..sub.g of gasoline (.epsilon..sub.g =2) and that .epsilon..sub.m of alcohol (incidentally, methanol in this case (.epsilon..sub.m =33)), namely, by first placing electrodes in the alcohol blended gasoline and thereafter measuring the electrostatic capacity between the electrodes.
FIG. 19 is a sectional diagram of an electrostatic capacity detecting portion C employed in the conventional device. In this figure, reference numeral 35 designates a metallic cylindrical housing which has a fuel inlet 33a and a fuel outlet 33b formed at opposite ends thereof, respectively. In this cylindrical housing (hereunder referred to simply as a housing) 35, a metallic internal electrode 31 is placed in such a manner that the longitudinal axis of the electrode 31 is coincident with that of the housing 35. A fuel passage is formed between the housing 35 and the internal electrode 31. Further, an electrode lead 36, around which a fuel seal 34 is wound, is connected to the internal electrode 31. Materials having high electric resistances are used for the housing 35, the internal electrode 31, the electrode lead 36 and the fuel seal 34. The internal electrode 31 and the housing 35 cooperate to form a capacitor. The electrostatic capacity C.sub.f of this capacitor changes according to the dielectric constant .xi. of the fuel which flows through the fuel passage formed between the internal electrode 31 and the housing 35.
The conventional device forms an LC parallel resonance circuit comprising a coil L and the capacitor C, which has an electrostatic capacity C.sub.f, by connecting the coil L to the electrostatic capacity C.sub.f in parallel with each other as an equivalent circuit illustrated in FIG. 20.
The dielectric constant s of fuel can be detected by detecting the resonance frequency of the LC parallel resonance circuit. In the case of the LC parallel resonance circuit of FIG. 20, the resonance frequency f.sub.0 thereof is obtained by the following equation (1): EQU f.sub.0 =1/.sqroot.2.pi.{L(C.sub.f +C.sub.p)} (1)
where L denotes the reactance of the coil; C.sub.f the electrostatic capacity between the internal electrode 31 and the housing 35; C.sub.p the sum of the floating capacity of the capacitor and the adjusting resistance of the LC parallel resonance circuit.
FIG. 21 shows the resonance-frequency versus methanol-mixing-ratio characteristic of the electrostatic capacity detecting portion C in the case of using methanol blended gasoline. As indicated by the curve C.sub.h, with increase in the methanol mixing ratio, the resonance frequency f.sub.0 monotonously decreases. For the simplicity of configuration, a back coupling oscillator, a Colpitts oscillator, a Hartley oscillator or the like is usually used as a circuit for detecting the methanol mixing ratio, so as to cause a parallel oscillation. Subsequently, the methanol mixing ratio is detected from an output signal of a frequency divider which divides the resonance frequency f.sub.0 of a signal obtained at the parallel oscillation, or from an output signal of a frequency-to-voltage (F/V) converter which performs a frequency-to-voltage conversion on the signal having the resonance frequency f.sub.0.
In the conventional methanol mixing ratio detecting device, an LC parallel resonance circuit is formed by connecting a coil to an electrostatic capacity detecting portion in parallel with each other. Further, the resonance frequency of this resonance circuit is obtained to thereby detect the dielectric constant of fuel, namely, the methanol mixing ratio. Methanol, however, has an affinity for water. Therefore, moisture is easily mixed into methanol blended gasoline. Thus, there is the possibility that the electric conductivity thereof increases in a region having a high methanol mixing ratio owing to the presence of various kinds of salts and ions mixed into the moisture.
Accordingly, in the region having a high methanol mixing ratio, with increase in electric conductivity of the methanol blended gasoline, there occurs a decrease in value of the resistance R.sub.f connected in parallel with the capacitor having the electrostatic capacity C.sub.f in the equivalent circuit of FIG. 20, thereby promoting the increase in the electric conductivity. Moreover, the Q-factor of the LC parallel resonance circuit decreases. Consequently, the conventional device fails to meet the oscillation condition. Thus, the conventional device has a problem that in such a case, the oscillation stops and it becomes unable to measure the resonance frequency.
Furthermore, even if the oscillation of the LC parallel resonance circuit does not stop, the oscillation thereof becomes unstable and the dielectric constant of water becomes large (.epsilon.=80). Thus, as indicated by the curve C.sub.l in FIG. 21, the values of the resonance frequency corresponding to the methanol mixing ratio are shifted from those indicated by the curve C.sub.h. Consequently, the conventional device has another problem that in this case, the methanol mixing ratio can not be achieved precisely.
A method for compensating an error in measurement of the methanol mixing ratio has been proposed and described in the Japanese Patent Laid-Open No. 2-213760/1990, in which the electric conductivity of the fuel is measured by providing another detecting electrode having a reduced electrode area and an increased resistance R.sub.f in an electrostatic capacity detecting portion and thereafter the alcohol mixing ratio is calculated from the measured electric conductivity and dielectric constant. This method, however, has drawbacks or problems in that it is necessary to provide the additional electrode in a housing and that thus the structure of a seal becomes complicated and the reliability of the electrostatic capacity detecting portion is deteriorated and the size thereof becomes large.